The invention relates to product dryers or ovens and in particular to air convection conveying dryers or ovens having a cooling section connected to a heating section.
Various types of products are dried, baked or toasted in convection type conveying dryers or ovens such as charcoal, pet foods, fish foods, foods for human consumption such as breakfast cereals and snack foods and other particulate type materials which may also include material in granular and flake forms. As used herein, the terms dryer and oven may be used interchangeably. A typical convection dryer includes one or more conveyer-driven product passes, convection or circulation fans, burners for elevating the temperature within a heating or drying section, and a cooling section. An upper conveyor may receive product to be dried, carry it the length of the drying section for a first product pass and then deposit it upon a lower conveyor that carries the product back through the length of the drying section for a second product pass. The cooling section is often constructed as an extension of the lower portion of the drying section that houses the second, lower conveyor in order to reduce fabrication costs and provide a more compact assembly, as compared to a separate dryer and a separate cooler. In such configurations, the lower conveyor is typically longer than the upper conveyer so that it may project into the cooling section. The conveyors are generally porous or perforated, with pores or openings sized large enough to permit heating and cooling air to pass through the conveyor and the bed of particulate material supported thereon, but small enough to prevent the particulate material from falling therethrough.
To initiate the drying process, product is introduced into the drying zone of the dryer and deposited on the conveyor. A process air stream in the drying section consists of a moving stream of heated air that removes moisture from the product as it is carried through the process air stream on the conveyor bed.
It is important that the temperature of the process air be controllable to avoid over or under heating of the process air which would lead, respectively, to detrimental effects on the product being dried or reduction in process efficiency and greater energy cost. In addition, it is important that the heated process air be contained in the dryer until exhausted through a dryer exhaust, and not lost to the cooling zone where it would hamper cooling of the product. During a steady state operating condition any lost or exhausted process air from the dryer must be compensated for through the introduction of freshly heated makeup air.
Dryers and other types of ovens are available with various airflow configurations, including some that cause air to flow upward through the product (air-up), some that cause air to flow downward through the product (air-down) and various combinations in which the air may flow both up and down through the product bed in different sections of the dryer depending upon the requirements of the end user and the product to be dried and cooled. The purpose of the convection or circulation fan or fans associated with the drying section is to force heated air through the product bed. Therefore, a positive pressure is exerted on one side of the product bed by the air flow from the fans and a negative pressure is created on the other side of the product bed.
Typically, a dryer inlet and transition section separates the drying section from the cooling section of the dryer. Cooling sections move fresh air through the dried product to cool it and are generally built in an air-down configuration, such that the air pressure beneath the product bed is less than the air pressure above the product bed during the cooling process. Baffles are usually installed in the transition section in order to minimize the amount of heated air that migrates from the drying section to the cooling section, but such baffles are only partially effective because the product bed is moving along a conveyor and the openings for the conveyor prevent forming of an effective seal between the drying section and the transition section.
Because the area beneath the product bed in the cooling section is typically at a relative negative pressure compared to the pressure beneath the bed in an air-up configured drying section, a large amount of heated air tends to migrate from the drying section to the cooling section. This unnecessarily increases fuel consumption in the heating or drying section.